There are a number of known types of jacks which are employed to lift a vehicle so that a tire can be changed. Jacks which engage the bumper of a vehicle were once popular but have been replaced by jacks which are adapted to engage the frame or other component of the vehicle because of automakers' change in the bumper design of modern vehicles. Typical of such frame-engaging jacks are the scissors jack and the screw jack, the latter being far more popular and reliable particularly for heavier vehicles such as pick-up trucks, vans and the like.
Of primary concern to the automaker is the cost, weight, effectiveness and safety of the screw jack. Since every vehicle is provided with a jack, and with the ever increasing costs of vehicles, the cost of the jack, which is passed along to the consumer, is important. Further, with emphasis on fuel efficiency, automakers are conscientious of every additional pound that goes into a vehicle, and thus the weight of the screw jack is a factor taken into consideration. The problem is, of course, that the desired lower cost and lower weight jack must be capable of safely raising, holding and lowering the vehicle.
Some of the costs associated with prior art screw jacks, and many of the safety factors, reside in the base thereof. In known screw jacks, the bottom stand thereof is attached to the base by a plurality of rivets or bolts. Such add to the cost of the product, both in material and labor.
Also, the base of some prior art jacks rests on the ground on the perimeter thereof which presents a negative safety factor. As the jack raises the vehicle, the vehicle inherently tilts which causes the jack to try to tilt. But because of its aforesaid position on the ground, prior jacks cannot tilt and the concomitant side loads on the jack become potentially dangerous.
Other safety and use problems in prior art screw jacks reside in the configuration of the screw assembly itself and its associated elements. These jacks have a cylindrical inner tube which vertically slides within a cylindrical outer sleeve during the process of raising the vehicle. However, these members must be prevented from rotating relative to each other as other screw components operatively attached thereto are turned. To prevent such relative rotation, the outer sleeve is normally provided with a vertical keyway slot and the inner tube is provided with a small key which rides in that slot. However, such keys and keyways are susceptible to breakage and/or distortion, particularly under high stress conditions, which results in not only a safety problem, but also renders the jack thereafter useless.
Another cost problem with prior art screw jacks is that the traditional bottom gear assembly requires that a ring be welded to the bottom gear. The ring fits inside of a bearing cup at the bottom of the jack and allows the screw assembly of the jack to effectively float to assist in the accommodation of side forces as it raises the vehicle. The use of such a ring, however, adds what has been found to be an unnecessary separate part and can require an additional manufacturing welding step.
Thus, the need exists for a screw jack which is lighter weight and less expensive to manufacture, both in parts and labor, and yet which is safer and more reliable than the prior art screw jacks.